As the inherent errors in the inertial navigation sensors (e.g., gyros and accelerometers) continue to be reduced, there arises another limitation on the accuracy that an inertial navigation system can achieve. This is the limitation caused by uncertainties in the earth's geopotential (i.e., gravitation field). Some investigators have concluded that the geopotential uncertainties are large enough to cause unbounded errors in even a high-quality unaided inertial navigation system, and to produce excessive errors in a velocity and/or position aided inertial system where precision navigation is required.
In terms of causing navigation errors, the most serious of the gravitational uncertainties is the uncertainty of the direction of the actual gravity vector with respect to the theoretical local vertical (the gravity deflection). The gravity deflection has been modeled by many investigators as a Markov process with an empirically-determined correlation function. To a moving vehicle, it is this random process that causes unacceptably large inertial navigation system errors.
During the past thirty years, the continued development of higher accuracy inertial components (gyros, accelerometers, computers, etc.), has made possible more and more accurate position, velocity and attitude outputs from the inertial systems carried in vehicles, and which use the 84 minute pendulum principle (Schuler tuning), or equivalent. As stated above, for several years now, it has become increasingly apparent that accuracy limits of both unaided, and of certain sets of aided inertial systems, is now limited by anomalies of the gravitational field and not by component inaccuracies.
Because of this anomaly limitation, many millions of dollars have been and are now being spent on attempts to remove this limit. For example, high accuracy satellite gravity surveys, electromagnetic radiation position and velocity fixing by means of ground based and/or air/sea based and/or active satellite systems.
Another proposed method of reducing the navigational errors caused by gravity deflection is to use a gravity gradiometer as a navigation aid. Such a device can be used to feed back real-time approximations of the gravity gradient into the navigation equations and thereby reduce the randomness associated with the gravity deflection.
Many of these approaches offer considerable improvement in accuracy but suffer from complexity, high cost, and vulnerability to intentional or unintentional external disturbances.
The present invention overcomes the aforementioned drawbacks by making use of the effects of the anomalies themselves to solve the problem. Specifically, the present invention makes use of the physical separation of multiple sensors mounted on a vehicle. The data from the multiple systems may be processed to reduce errors caused by the uncertainty in the gravity deflection without the need of any additional instruments.